Heart failure (HF) is well known for its deleterious effects on functional performance and quality of life, and this problem has been persistent despite advances in pharmacological and other treatments. Sleep disturbance, including deceased quantity, fragmentation, and poor self-reported quality, and sleep disordered breathing (Cheyne-Stokes breathing/central apnea and obstructive sleep apnea) are common in HF patients and there is beginning evidence that both sleep and SDB contribute to functional performance. However, the bio-behavior attributes of sleep in HF patients, the contribution of sleep disturbance and SDB to function performance, and the behavioral pathways through which this may occur have not been well explained. Therefore, the purpose of this study is to explicate the nature of sleep disturbance and its contributions to functional performance in a well-defined group of stable HF patients. The specific aims are to: 1. Characterize the physiologic, behavioral, and subjective attributes of sleep in HF patients; 2. Evaluate the contributions of sleep disturbance and SDB (CSB-CSA and OSA) in functional performance of HF patients; 3. Examine the relationships among nocturnal symptoms, sleep disturbance and SDB; and to (1) mood disturbance; (2) excessive daytime sleepiness, and (3) fatigue in HF patients; 4. Evaluate the extent to which fatigue, excessive daytime sleepiness, and mood disturbance mediate the contributions of sleep disturbance to functional performance in HF patients; and 5. Evaluate the influence of age and gender on the relationships between sleep disturbance and functional performance in patients with HF. This project will use a multivariate cross-sectional design and enroll a sample of 400 male and female stable HF patients. Acquisition of sleep and functional performance data in participants' homes, using physiologic, behavioral, and self-report measures, will provide an ecologically valid perspective on sleep and its contribution to functional performance. The use of nocturnal polysomnography will permit a comprehensive examination of sleep itself, as well as sleep disordered breathing. Hypothesized mediator variables will include excessive daytime sleepiness, fatigue, and mood disturbance, measured with well-validated instruments. The potential influences of age, gender, selected clinical variables, and the role of primary sleep disorders (e.g. Cheyne-Stokes breathing and obstructive sleep apnea) on the sleep and the functional performance variables in a multivariate model will be addressed. Multiple regression analysis and structural equation modeling (SEM) will be the primary approaches to data analysis. SEM will be used to evaluate the factor structure of the sleep disturbance construct and serve as a tool to explore the direct and indirect pathways linking the major study variables.